Controlling a polarization mode and a spatial mode of optical signals in photonic integrated circuits (PICs) is important for optical communication networks. For example, a conventional single mode fiber does not preserve the polarization mode. When the optical signal is coupled from the single mode fiber to the PICs, the signal decomposes into arbitrary compositions of two orthogonal polarization components, namely, a first component in a transverse electric (TE) mode and a second component in a transverse magnetic (TM) mode. In many modules used in the PICs, the components in the TE and TM modes have different characteristics. For example, the components having different TE and TM modes propagate at different velocities in a high index contrast waveguide, and the energy coupling coefficients of a microring resonator for the TE and TM modes are different.
These polarization-dependent effects reduce the performances of the PICs, especially for high speed communication. Also, most optical communication networks use only one polarization mode. Furthermore, if the components in both polarization modes are used in polarization-division multiplexing (PDM) systems, then the spectral efficiency of such systems can be increased.
Typically, systems for controlling polarization of optical signals, e.g., polarization transparent systems and polarization multiplexing systems, use various polarization manipulators, such as polarization converters and/or polarization splitters. For example, polarization splitters can be utilized in polarization transparent systems to solve, e.g., polarization dependence and polarization mode dispersion problems in the current photonic integrated circuits (PICs). Also, the polarization splitters can be utilized in polarization-division multiplexing (PDM) systems to increase the spectral efficiency.
Accordingly, there is a need to design a polarization manipulator that is compact, has a large bandwidth, and simple in fabrication.